In 3rd Generation Partnership Project (3GPP) which is a mobile communication system standardization project, standardization of a device to device proximity service (D2D ProSe) is in progress.
Two schemes including “sidelink direct discovery” and “sidelink direct communication” are specified as a scheme for the “D2D ProSe.” Here, the sidelink direct discovery is a scheme of searching for a counterpart by directly transmitting a discovery signal (a sidelink signal) not designating a specific destination between radio terminals. The sidelink direct communication is a scheme of designating a specific destination group and directly transmitting data (sidelink signal) between radio terminals.
For the sidelink direct discovery, inter-frequency discovery monitoring is specified. In inter-frequency discovery monitoring, a radio terminal monitors (and receives) the discovery signal at a frequency different from a frequency of a serving cell. On the other hand, inter-frequency discovery transmission (inter-frequency discovery announcing) is not specified, and transmission of the discovery signal is limited to the frequency of the serving cell.